The conventional adjustable wrenches, such as those which are disclosed in the U.S. Pat. Nos. 1,577,789 and 1,500,314 and the British Patent Nos. 10,112 and 160,513, are defective in design in that the clamped piece, such as a hexagonal nut, is prone to slip away from the jaws of the wrench when the handle of the wrench is exerted on by a force.